The present invention relates to a pump used suitably for supplying of water to a boiler, and particularly, to a single-shaft multistage pump having a structure in which impellers for pressurizing drawn water are mounted at multiple stages on a single rotary shaft.
A conventional typical structure of a single-shaft multistage pump used, for example, for supplying water to a boiler is designed as shown in FIG. 6 (for example, see JP-A-2002-242881 and JP-A-2001-248586). As can be seen in FIG. 6, the single-shaft multistage pump includes a rotary shaft 1, impellers 2, a radial bearing 3 for a suction port-side end, a radial bearing 4 for a discharge port-side end, a thrust bearing 5 for the discharge port-side end, a seal device 6 for the suction port-side end, a seal device 7 for the discharge port-side end, and a balance device 8. The single-shaft multistage pump also includes a housing 10 in which the rotary shaft 1, the impellers 2 and the like are accommodated. The housing 10 comprises a suction casing 12 provided with a suction port 11, a stage 13 in which the impellers 2 are accommodated, a discharge casing 15 provided with a discharge port 14, and a stuffing box or a seal box 16.
The rotary shaft 1 is connected through a spacer S and a coupling C directly to an output shaft of a drive unit D, so that it is rotated under reception of a rotational driving force from the drive unit D in a state in which it is supported at its suction port-side end and its discharge port-side end in a radial direction by a radial bearing 3 and a radial bearing 4 each of which is a sliding bearing, respectively, and also supported at its discharge port-side end in a thrust direction by a thrust bearing 5. The radial bearings 3 and 4 and the thrust bearing 5 are generally oil-lubricated bearings, respectively. In the case of the oil-lubricated bearing, the bearing is supplied with a lubricating oil from a hydraulic system (not shown). The impellers 2 are disposed at a plurality of stages on the rotary shaft 1, so that they are rotated with the rotation of the rotary shaft 1 to pressurize supplied water W drawn from the suction port 11 sequentially at the individual states to a predetermined pressure. Then, high-pressure water Wh pressurized to the predetermined pressure is discharged from a discharge port.
In a course of pressurizing the water by the impellers 2, a thrust force (an impeller thrust force) in a sideways direction of a suction port is generated axially on the rotary shaft 1. The balance device 8 is mounted in order to overcome the impeller thrust force. The balance device 8 is comprised of a balance member 21 which is formed into a disk shape (a shape in an example in FIG. 7) or a drum shape, as shown in an enlarged scale in FIG. 7, and which is fixedly mounted to the rotary shaft 1, an intermediate chamber 22 defined in a surface of the balance member 21 on the side of the suction port, and a balance chamber 23 defined in the surface of the balance member 21 on the side of the discharge port. The intermediate chamber 22 is in communication with the discharge port 14 through a very small clearance (not shown) formed along an outer periphery of a boss portion 21b of the balance member 21, so that a high pressure is generated in the intermediate chamber by the high-pressure water Wh. On the other hand, the balance chamber 23 is in communication with the suction port 11 through a balance pipe 24 and also with the intermediate chamber 22 through a very small clearance (no shown) intended to be changed in clearance width in association with the magnitude of the impeller thrust force, so that a pressure in the balance chamber 23 (this pressure is changed in accordance with the change in above-described clearance width) is lower than the pressure in the intermediate chamber 22. This brings about a state in which the disk portion 12b of the balance member 21 is urged in a direction toward a discharge port-side portion of the rotary shaft 23, whereby a thrust force (a balance thrust force) opposing the impeller thrust force is applied to the rotary shaft 1. As a result, it is possible to ensure that the thrust force generated on the rotary shaft 1 is smaller, or no substantial thrust force can be generated on the rotary shaft 1. In a case where no substantial thrust force can be generated on the rotary shaft 1 by virtue of such balance device 8, the thrust bearing 5 is not necessarily required, and may be omitted in some cases.
Each of the seal devices 6 and 7 is configured by a mechanical seal. More specifically, the seal device is of a structure in which the sealing is achieved by the sliding contact of the rotary ring 25 fixedly mounted to the rotary shaft 1 with the stationary ring 26 retained in a fixed state in the stuffing box or the seal box 16. The seal devices 6 and 7 serve to prevent water from being leaked to the outside along the rotary shaft 1, while also serving at the same time to prevent water from entering to the oil-lubricated radial bearings.
There are the following problems for the single-shaft multistage pump as described above: One of the themes is a reduction in size and a space-saving. A single-shaft multistage pump is increased in its axial size, because impellers are mounted at multiple stages on a single rotary shaft. For this reason, it is desired that the axial size reduced as much as possible to provide a space-saving in a pump system. With regard to this, for example, in each of the single-shaft multistage pumps disclosed in JP-A-2002-242881 and JP-A-2001-248586, a water-lubricated bearing is intended to be used for the bearing. As a result of the use of water-lubricated bearing, the seal device (the seal device 7 in FIG. 6) mounted for sealing the bearing against water is not required, and thus, a reduction in axial size can be realized, thereby providing a space-saving and further, a hydraulic system for supplying a lubricating oil is not required, whereby the arrangement around the pump can be simplified, which also provides a space-saving. In addition, in each of the single-shaft multistage pumps disclosed in JP-A-2002-242881 and JP-A-2001-248586, by employing the structure comprising the balance device and the thrust bearing integral with each other, one of the balance device and the thrust bearing is not required, whereby a reduction in axial size can be realized to provide a space-saving.
The use of the water-lubricated bearing for the bearing as in the techniques disclosed in JP-A-2002-242881 and JP-A-2001-248586 is effective for the reduction in size and the space-saving of the single-shaft multistage pump. In these prior arts, however, the water-lubrication of the bearing causing the seal device for the bearing to be not required is employed usefully only for the reduction in size of the single-shaft multistage pump, but there is still an unsatisfactory respect.
As described in JP-A-2002-242881, a water-lubricated carbon bearing is conventionally employed as the water-lubricated bearing. The carbon bearing is formed by sintering a carbon material and suffers from a problem that it is poor in shock resistance, because it is hard and brittle. This problem is particularly severe in the single-shaft multistage pump. More specifically, in the single-shaft multistage pump, the impellers are mounted at the multiple stages on the single rotary shaft and for this reason, the rotary shaft is longer and hence, a span between the bearings is longer. Therefore, the rotary shaft is liable to be brought into one-side collision against the bearings due to an increase in amount of rotary shaft flexed by its own weight and due to the whirling of the rotary shaft caused by a change in motional state. If the one-side collision occurs in a state in which a lubricating water film is not formed sufficiently on a slide surface of the bearing as at the start of the pump, the carbon bearing poor in shock resistance may be damaged in some cases. In addition, because the viscosity of water as a lubricant for the water lubrication is lower than that of an oil for the oil lubrication, the possibility of the damage to the bearing is increased, and the carbon bearing is defective in reliability and difficult to handle. Further, the carbon bearing is also accompanied by a problem concerning a sliding clearance (a clearance between an inner surface of the bearing and an outer surface of the rotary shaft). More specifically, the accuracy of the sliding clearance is important to form a sufficient water film, because of the low viscosity of water as the lubricant. However, the carbon material has a expansion coefficient larger than that of the rotary shaft and for this reason, the sliding clearance may be excessively widened in some cases due to a rise in temperature of the bearing after the start of the pump, whereby a sufficient water film may be not formed. In such a case, a solid lubrication occurs due to the absence of the water film, thereby causing the wearing of the bearing to be hastened.
The other problem resides in a maintenance regarding the seal device. It is usual to use the mechanical seal for the seal device, as described above. However, the mechanical seal is a component wasted earliest among various components, and hence, the frequency of the maintenance service and inspection is correspondingly increased. In the maintenance service and inspection of the seal device, it is necessary to remove the rotary ring and the stationary ring of the seal device in order to examine the worn state of the sliding surface. To carry out the operation for removing the rotary ring and the stationary ring in the conventional cylindrical seal device comprising the rotary ring and the stationary ring integral with each other, a operating procedure is required, for example, for the seal device for the suction port-side end, which comprises first removing a spacer and a coupling used to connect a drive unit and the rotary shaft to each other, thereby providing a state in which the end of the rotary shaft is open, and then withdrawing the rotary ring and the stationary ring along the rotary shaft. To inspect the seal device, it is necessary to disassemble even the bearings which are not required to be inspected. For this reason, a lot of time is required for the maintenance service and inspection, and thus, an improvement in maintenance property has been demanded.